This invention relates to a method and apparatus for controlling the zero adjustment of a weighing machine used in a combinatorial scale.
It is often required to accurately weigh articles, which have unit weights that differ from one to another, into fixed quantities while at the same time limiting the articles in number. Examples of such articles are vegetables and fruits, confectioneries, perishables, fabricated articles, etc. When weighing out such articles it is general practice to make use of a combinatorial scale that relies upon a computer. A scale of this type is referred to as a computer scale.
A computer scale, as illustrated in FIG. 1, is composed of a plurality of weighing machines A.sub.1, A.sub.2, . . . , A.sub.n, weighing hoppers B.sub.1, B.sub.2, . . . , B.sub.n associated with respective ones of the weighing machines, a common guide chute C, a pool hopper D, and buckets E operatively associated with a packing machine or the like.
In the computer scale of this kind each of the weighing hoppers B.sub.1, B.sub.2, . . . , B.sub.n are supplied with articles to be weighed. The articles in each hopper are then weighed, and different combinations of the weighed values are added in each weighing cycle, with the number of weighed values in each combination being either arbitrary or predetermined. The sum of the weights in each combination is compared with a set target weight, whereby the weight combination that gives the target weight or the value closest thereto is found for each weighing cycle. This combination of weights is referred to as the best combination. When the best combination is found in the manner described, only those weighing hoppers of the weighing machines that are specified by the best combination (in other words, the best combination of weighing hoppers) are opened in response to an electric signal, thereby discharging their contents into the common guide chute C which in turn guides the articles to the pool hopper D where they are collected. The pool hopper D is subsequently actuated to introduce the articles into the bucket E which carries the articles to the packing machine or the like.
In weighing articles using the aforementioned computer scale, a wide variety of matter such as powder, oil, salt and other residue orginating from the weighed articles attaches itself to the walls of the weighing hoppers B.sub.1, B.sub.2, . . . , B.sub.n. While the amount of such matter left clinging to a weighing hopper is extremely small for one weighing operation by that hopper, the total amount which can accumulate over a large number of weighing operations is significant. Accordingly, a computer scale incorporates an automatic zero adjustment circuit which automatically applies a zero adjustment to the weight sensors associated with the respective weighing hoppers. When a preset number of weighing operations have been performed, the automatic zero adjustment circuit is adapted to automatically adjust the zero point of the weight sensor belonging to the weighing hopper which has performed said preset number of weighing operations. The zero adjustment referred to here is for the purpose of setting the output value of a weight sensor to zero when the corresponding weighing hopper is empty. This output value of the weight sensor when the hopper is empty is referred to as the zero point deviation.
Often the articles which are to be weighed exhibit a viscose or sticky property and have a relatively large unit weight namely the weight of an individual article, such as pieces of uncooked meat. In weighing such articles there are instances where several pieces or portions thereof attach themselves to the walls of a weighing hopper, as occurs with the various residue mentioned above.
The weight of the discharged articles to be weighed is light because a portion is attached to and remains within the weighing hopper despite the fact that the computer scale has selected the best combination.
On the other hand, some of the residual articles remaining in the weighing hopper are present in the form of a mixture of articles so firmly attached that they are rarely peeled off by the conventional weighing operation of the aforesaid powder, oil and fats, gas, etc. (hereinafter referred to as gases) but are finally peeled off by cleaning agent, brushing or the like, and another part of the articles to be weighed remain attached without being discharged for a period of time due to a slight viscosity of the articles to be weighed, as described above.
In such a case, in the prior art method for controlling the zero adjustment, if the weighing hopper, in which the residue articles to be weighed remain, reaches the time for the zero adjustment, the automatic zero adjustment is effected. The output value of the weight sensor in the weighing hopper after discharge with the attached residue is subjected to the zero adjustment and becomes the weight of an empty hopper, that is, the amount of zero displacement is set, without discriminating whether the gases or attached residue is hard to peel off after the articles to be weighed have been discharged. If the weighing hopper whose weight sensor is zero-adjusted has an article or portion thereof clinging to it, weight, too, can be sensed correctly by the sensor only as long as the article remains attached. Since the clinging article's residue is comparatively heavy, and the viscosity slight however, it will almost never remain attached to the hopper permanently but will usually fall after the following weighing operation together with the other articles which are released from the hopper. It is obvious that an error will result when the zero adjustment is applied with articles or fragments thereof clinging to a weighing hopper, followed by the dislodging and dropping of said articles or fragments into the chute C. Specifically, the output of the weight sensor belonging to the weighing hopper from which the clinging matter has fallen will include an error w.sub.o which will prevail until the next zero adjustment, the error w.sub.o representing the weight of the formerly attached matter or residue. With the conventional zero adjustment method, in other words, there are cases where weighing cannot be performed with a high level of accuracy when the articles to be weighed remain in the hopper.